Thanks for the fast response. I get these errors, when I do that: Argument of \@citex has an extra }. Runaway argument? page 10\color@endgroup Latex Error: Paragraph ended before \@citex was complete. Latex Error: Extra }, or forgotten \endgroup.
– user46365May 1 '11 at 19:01

It must be some other package which is causing troubles :\
– user46365May 1 '11 at 19:03

Tried it on a fresh file and it worked like a charm.. bummer.
– user46365May 1 '11 at 19:04

@user46365: What packages that might affect theorems do you use (amsthm, ntheorem)? What document class? A working example would be even better.
– lockstepMay 1 '11 at 19:04

@user46365: Strange enough, my example still works if I load amsthm, but not with ntheorem. Obviously you don't use ntheorem if it works for you now.
– lockstepMay 1 '11 at 19:07

Welcome! This answer has already been given by others, please make sure that your answer is really a "new answer" before clicking that button. Small improvements on existing answers can be suggested in the comments or with the "edit" button.
– T. VerronSep 29 '14 at 14:19